Similiar to my situation, I had just sent off a full stack and a half to the indus and then bam, Seleukids and Parthians betray me and its all over. As for Baktrian battle tactics, I much prefer a hammer and anvil style, or pike-flankers with a weak center and a general behind it. Draws the enemy right into a killing zone, just support your pike flankers with some light cavalry.